Faint galaxies, extragalactic background light, and the reionization of the Universe

I review recent observational and theoretical progress in our understanding of the cosmic evolution of luminous sources. Largely due to a combination of deep HST imaging, Keck spectroscopy, and COBE far-IR background measurements, new constraints have emerged on the emission history of the galaxy population as a whole. Barring large systematic effects, the global ultraviolet, optical, near- and far-IR photometric properties of galaxies as a function of cosmic time cannot be reproduced by a simple stellar evolution model defined by a constant (comoving) star-formation density and a universal (Salpeter) IMF, and require instead a substantial increase in the stellar birthrate with lookback time. While the bulk of the stars present today appears to have formed relatively recently, the existence of a decline in the star-formation density above z=2 remains uncertain. The history of the transition from the cosmic `dark age' to a ionized universe populated with luminous sources can constrain the star formation activity at high redshifts. If stellar sources are responsible for photoionizing the intergalactic medium at z=5, the rate of star formation at this epoch must be comparable or greater than the one inferred from optical observations of galaxies at z=3. A population of dusty, Type II AGNs at z<2 could make a significant contribution to the FIR background if the accretion efficiency is of order 10%.Comment: LateX, 13 pages, aipproc.sty, 5 figures. To appear in the proceedings of the 9th Annual October Astrophysics Conference in Maryland, ``After the Dark Ages: When Galaxies were Young", edited by S. S. Holt and E. P. Smit

Cosmic Star Formation History

I review some recent progress made in our understanding of galaxy evolution and the cosmic history of star formation. Like bookends, the results obtained from deep ground-based spectroscopy and from the Hubble Deep Field imaging survey put brackets around the intermediate redshift interval, $1<z<2$, where starbirth probably peaked at a rate 10 times higher than today. The steady decline observed since $z\sim 1$ is largely associated with late-type galaxies. At $z\gtrsim 2.5$, the Lyman-break selected objects may represent the precursors of present-day spheroids, but appear, on average, quite underluminous relative to the expectations of the standard early-and-rapidly forming picture for spheroidal systems. The observed ultraviolet light density accounts for the bulk of the metals seen today in ``normal'' massive galaxies.Comment: 10 pages, 3 figures, LaTeX with AIP "aipproc" style file. Review Presented at the 1996 7th Annual October Astrophysics Conference in Maryland, ``Star Formation Near and Far'

Technical Efficiency in Organic Farming: an Application on Italian Cereal Farms using a Parametric Approach

A stochastic frontier production model was applied to estimate technical efficiency in a sample of Italian organic and conventional cereal farms. The main purpose was to assess which production technique revealed higher efficiency. Statistical tests on the pool sample model suggested that differences between the two cultivation methods were significant from a technological viewpoint. Separate analyses of two sub-samples (93 and 138 observations for organic and conventional farms, respectively) found that conventional farms were significantly more efficient than organic farms, with respect to their specific technology (0.892 vs. 0.825). This implies that organic (conventional) cereal farmers could increase their income to 99.19 €/ha (40.95 €/ha). Analysis also estimated that land was the technical input with the highest elasticity for both technologies. Furthermore, findings indicated that 63.7% of the differentials between observed and best-practice output was explained by technical inefficiency for the conventional group, while this value was close to unity for organic farms. Some policy implications can be drawn from these findings